1. Field of the Invention
The present invention relates to the detection of arcing faults at components of electric power systems and, more particularly, to detecting such faults within the metal cabinets of switchgear. More specifically, the invention relates to an apparatus and method for detecting arcing faults with optical fibers.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems, which form part of the overall electric power system, include main and branch power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within switchgear cabinets, such as between power buses, or between a power bus and a grounded metal component. Such arcing faults can produce high energy gases which pose a threat to the structure and nearby personnel. This is especially true when maintenance is performed on or about live power circuits. Frequently, a worker inadvertently shorts out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.
A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional costs due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear cannot be contained.
Recently, methods have been developed to minimize the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection, which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage.
The light from the arcing fault contains wavelengths characteristic of the material creating the arc. Commonly, the arcing fault occurs at a power bus or disconnect, which are typically made of copper. Copper has a strong line emission wavelength at about 520 nm (e.g., 520.820 nm). In some applications, the power buses are coated with silver, which also has a strong line emission characteristic wavelength of about 520 nm (e.g., 521.908 nm) and another at about 546 nm (e.g., 546.550 nm). Unfortunately, other light sources (e.g., ambient light; tungsten bulb light; flashlight; fluorescent light; flash bulb light), which can be present, have a broadband continuum of wavelengths, which includes 520 nm and 546 nm. Thus, merely detecting light at about 520 nm will not distinguish an arcing fault from these other sources.
U.S. Pat. No. 6,229,680 discloses the detection of an arcing fault by gathering light from components susceptible to arcing faults and splitting this light into two beams. Light within a narrow band of wavelengths, which includes a wavelength characteristic of arcing from the material of the component, is extracted from the first beam by a first narrow beam sense filter (e.g., 520 nm to detect the characteristic emission lines of copper and silver). Light from a second narrow band of wavelengths, not including the characteristic wavelength for the arcing fault, is extracted from the second beam by a second narrow band background filter (e.g., 610 nm). Preferably, the second band of wavelengths is selected to have a wavelength range that has a greater intensity of the background light passed by the second or background filter than by the first or sense filter. Hence, in the absence of an arcing fault, the light passed by the background filter will be greater than that passed by the sense filter. However, when an arcing fault is present and light is generated by the arcing fault at the characteristic wavelength of the arcing material, the output of the sense filter will exceed that of the background filter and can be used as the indication that an arcing fault is present.
U.S. Pat. No. 5,650,902 discloses a device to detect an arcing fault in the bus bar compartment of a low voltage substation. The device includes an optical fiber conductor with a colored acrylate jacket, an electronic circuit with a light-emitting diode, which emits a constant light beam of a defined wavelength at the beginning of the optical fiber conductor, and a receiver on the end of the optical fiber conductor. This light beam is used to monitor protective equipment. If an arcing fault develops in the bus bar compartment, then the light from the fault is injected into, or interferes with, the optical fiber conductor through its jacket. The electronic circuit generates a voltage, which is proportional to the level of light. This additional light essentially raises the level of light received by an evaluation circuit. After a specified make-and-break level (e.g., as set by the evaluation circuit) is exceeded, a signal is generated for use by a selective protective device to deactivate the portion of the substation in which the arcing fault is occurring, or another suitable device. See, also, U.S. Pat. No. 5,940,547.
U.S. Pat. No. 5,650,902 also discloses that the optical fiber conductor is oriented perpendicular to bus bars and is wound therearound, without touching them. The optical fiber conductor is wound around each bus bar several times, either over the entire length of the bus bar or over major portions of it. Although there is an increased danger that the optical fiber conductor may be destroyed, the destruction will occur after the fault has been detected. Alternatively, the optical fiber conductor is arranged in a meandering fashion in front of, behind or parallel to bus bars at an approximately uniform distance; wound around connecting bars of field bus bars; and wound between current taps in an area where arcing faults will most probably occur.
It is known to employ a high-speed shorting switch, placed between a power bus and ground, or from bus to bus, in order to limit or prevent equipment damage and personnel injury due to arc blasts. It is also known to employ various types of crowbar switches for this purpose. The switches short the line voltage on the power bus, eliminating the arcing fault and preventing damage. The resulting short on the power bus causes an upstream circuit breaker to clear the fault. See, for example, U.S. Pat. No. 6,633,009.
Such shorting switches, or other known shorting switches, may be applied in low voltage (e.g., up to about 690 VAC) and/or medium voltage (e.g., about 1 kV to about 38 kV) applications. For example, FIG. 1 shows medium voltage (e.g., 15 kV/60 MVA with a 50 kA fault potential) switchgear 2 for a three-phase power source 4. Associated with a three-phase power bus 6 is a first shorting switch 8, which is disposed between phases A and B, and a second shorting switch 10, which is disposed between phases B and C. Although the three-phase switchgear 2 and power source 4 are shown, one of the shorting switches 8,10 may be applied in a single-phase application. Although phase-to-phase shorting switches 8,10 are shown, such shorting switches may be applied from phase to ground 12. Disposed within the switchgear 2 are a plurality of light sensors 14,16,18,20, which detect the presence of arc light 22 associated with an arcing fault 24. In response to the arcing fault 24, one or more of the sensors 14,16,18,20 detect and communicate the presence of the arc light 22 to a trigger/power circuit 26, which responsively sends an actuation signal 28 to one or both of the shorting switches 8,10.
There is a need, therefore, for improved apparatus and method for detecting arcing faults in electric power systems and, particularly, within switchgear.
There is room for improvement in apparatus and method for detecting arcing faults.
There is also room for improvement in apparatus and method for protecting an electric power system bus from arcing faults.